1. Field of the Invention
The present invention relates to a projection exposure system which may be used in a method of manufacturing miniaturized devices and to a corresponding method.
2. Brief Description of Related Art
Lithographic processes are commonly used in the manufacture of miniaturized structures such as integrated circuits, liquid crystal elements, micro-patterned structures and micro-mechanical components.
A projection exposure apparatus used for photolithography generally comprises a projection optical system for imaging a patterning structure, commonly referred to as a reticle, onto a substrate commonly comprising a semiconductor wafer and a photo-sensitive layer, commonly referred to as a resist, which is exposed with the image of the patterning structure using imaging light. The imaging light is generated by an illuminating optical system illuminating the patterning structure.
The trend to produce even smaller and more sophisticated miniaturized devices places increasingly high demands on the projection exposure system. In particular, an optical performance of this system has to be maintained during the exposure process, such as subsequent exposures of plural dies on the wafer. The quality of the optical imaging may be controlled by exposing a resist-coated wafer with a predetermined test pattern, developing the resist and detecting the developed resist pattern by using a microscope such as a scanning-electron microscope. Deficiencies in the detected pattern may be analyzed and optical components of the projection exposure system may then be adjusted to obtain a desired imaging performance of the system. Such process is complicated and takes too much time to be used frequently in the manufacture of miniaturized devices.
From US 2003/0128346 A1, which is cited based upon preliminary understanding and without any formal admission of prior art, there is known a projection exposure system which allows to verify some characteristics of the optical performance without analyzing an exposed and developed test pattern. The system comprises a Fizeau interferometer including a Fizeau lens which may be inserted in a beam path of the imaging light upstream of the reticle. A wafer stage has a retro-reflector incorporated therein, wherein the retro-reflector is disposed adjacent to the wafer to be exposed and mounted on the wafer stage. For testing the optical performance of the projection optical system, measuring light is directed through the Fizeau lens and the projection optical system to be incident on the retro-reflector. The measuring light reflected from the retro-reflector travels back through the projection optical system and the Fizeau lens to be superimposed with measuring light reflected from the Fizeau lens to generate an interference pattern which is detected by a detector. The interference pattern is indicative of the optical performance of the system.
Since the wafer-stage has to be displaced by such an amount that the wafer is moved to the outside of the field of view of the projection optical system and that the retro-reflector is positioned within the field of view, a significant interruption of the exposure process is necessary to determine characteristics of the imaging quality with the conventional system. Moreover, the conventional system is difficult to be applied to immersion-type projection exposure systems having an immersion liquid disposed between the wafer surface and a front lens of the projection optical system.